lightnvm: ensure that nvm_dev_ops can be used without CONFIG_NVM

commit a7fd9a4f3e8179bab31e4637236ebb0e0b7867c6 upstream.

null_blk defines an empty version of this ops structure if CONFIG_NVM
isn't set, but it doesn't know the type. Move those bits out of the
protection of CONFIG_NVM in the main lightnvm include.

Signed-off-by: Jens Axboe <axboe@fb.com>
[pebolle: backport to v4.4]
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>